It is a regular problem for persons who have vacated premises for what may be a relatively long period of time to be certain that a freezer located at such premises has not failed during that time period with the result indicated above.
Freezer failure of the type under consideration can have extremely dire consequences in that frozen food may have become partially or completely thawed for some period of time during which it could develop harmful bacteria or the like followed by re-freezing once the freezer is functional once more. Persons could thus unknowingly consume tainted food and become poisoned by it.
Cold storage and other commercial freezers are usually monitored continuously and a record maintained of the temperature of the freezer so that compliance with cold storage specifications can be checked and authenticated.
However, at a domestic level, and even at the level of small freezers in retail outlets, no such record is maintained and it is not always easy to assess the extent of de-frosting that may have taken place prior to the freezer functioning once more. The condition of frozen foods or other perishables that have the potential to become spoiled is often not known.
Many prior art devices have been proposed to indicate when a freezer has failed to the extent that frozen food stored in the freezer may have thawed to an undesirable extent. Some of these are based on the principle that a solid body rests on or in a body of frozen liquid such that if the liquid thaws the solid body will fall to the bottom of the liquid container and will be easily visible to indicate that thawing has occurred. British Patent No 2,235,969 is an example of a device of this type whilst U.S. Pat. No. 4,144,532 is an example of a more sophisticated version of this type of device in which the weight closes an electric circuit when it falls to the bottom. A reverse principle is employed in U.S. Pat. No. 4,064,828 in which a buoyant indicator is locked within a frozen liquid and if the liquid thaws the buoyant indicator rises.
Other devices simply operate on the locking of a frozen body of liquid in an upper position such that it flows downwards, or in one case laterally, if thawing takes place. The frozen body of liquid is generally created by freezing the liquid in the device with the latter in one orientation and then subsequently inverting the device so that the frozen body of liquid is then uppermost. Some of the devices choose to select liquids with different freezing points, and indeed some use multiple liquid indicators with different freezing points to indicate the degree of the freezer failure. Examples of these devices are German Patents 3,940,163 and 4,418,927 and U.S. Pat. Nos. 4,114,443 and 4,941,425.
Other devices such as that described in European published patent application EP 0 740 136 A1 and applicants' own international patent application published under No WO 01/73388 A1 describe indicators that utilize a non-freezing liquid, in particular an organic liquid such as paraffin, and an immiscible aqueous liquid. With the aqueous liquid frozen, and the indicator inverted, the organic liquid which can typically be coloured green is visible in the lower region of the indicator and in the event of thawing taking place, the aqueous liquid that is typically coloured red replaces it and becomes visible to indicate the failure.
All of these devices are more or less relatively complicated, difficult to manufacture, or do not clearly indicate the failure because they employ a single colour or solid object with only its position changing. In the case of our earlier international patent application the problem of the use of a single colour was overcome by the use of two immiscible liquids. However, this proved to be inconvenient because the most advantageous and non-toxic organic phases were found to exhibit an unsatisfactory partition coefficient, at least after a significant period of time. Also, assembly of the particular embodiment of indicator described in that earlier patent application proved to be difficult in view of the physical and chemical properties of the organic phase.
An additional difficulty with existing proposals, and one that is not dealt with in our earlier application, is the fact that the container of the type of device with which the invention is concerned generally has to be pre-frozen in one orientation and then inverted in order to be rendered functional. It is not always immediately apparent, or, alternatively, not easily perceived, that the indicator has not been inverted from its position in which it becomes pre-frozen to the position in which it becomes functional.